The other computer is running a Kubuntu LiveCD where I started nc -u -l 6666.
And if I start the (first) computer with 3th grub option, I can run nc -u 192.168.0.27 6666 to communicate with the 2nd computer.

The difference between /boot/linux-3.0.6-gentoo and /boot/linux-3.0.6-gentoo-debug is that I enabled:

The netconsole looks fine to me, but I take it that it doesn't work for you. Not sure why. Are you sure that the driver for your NIC is compiled into the kernel, not loaded as a module? I have the experience that serial console might be more robust to set up. So if your computers are old enough to have serial ports and if you have a nullmodem cable, or feel like building one, then that might be an alternative.
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MvGJul 11 '12 at 19:13